The present invention relates generally to inspection systems. More specifically, it relates to light collection mechanisms for inspecting semiconductor wafers and other types of patterned samples.
Conventional darkfield optical inspection tools locate defects on patterned wafers by scanning the surface of the wafer with a tightly focused laser spot and measuring the amount of light scattered by the illuminated spot on the wafer. Dissimilarities in the scattering intensity between similar locations in adjacent dies are recorded as potential defect sites.
The dynamic range of this optical scattering is typically substantial. Changes in scattering intensity of more than a million to one within a single die are not uncommon. This high dynamic range is intrinsic to the optical configuration of the instrument and the scattering properties of the wafers and defects of interest. Because this dynamic range is substantially greater than the reliable measurement range of existing instruments, inspection operators are forced to accept an unpleasant compromise between inspecting with too low a sensitivity in some portions of the die, and temporarily overloading the instrument""s detection electronics in other regions.
In general, scanning the wafer with the smallest possible laser spot size maximizes sensitivity to defects by maximizing the spatial resolution of the scattering image. However, this increased resolution generally correlates with an increased pixel density within the light collectors or detectors to properly sample the image. The detectors typically include a sensor for detecting the scattered light and generating an analog signal based on such detected light and an analog-to-digital converter (ADC) for converting the analog detected signal into a digital detected signal. The digital detected signal may then be analyzed for defects. Since all the pixels are measured serially, and only a limited amount of time is available to scan each wafer, there is a fundamental relationship between the speed of the measurement electronics and the maximizing of the spatial resolution of the scattering is image. To enable high spatial resolution, higher bandwidth analog electronics and faster ADC""s are often utilized.
In addition to maximizing the speed of the measurement electronics to thereby maximize spatial resolution, it is desirable to maximize the dynamic range of the light that is discernable by the measurement electronics. However, there is a fundamental trade-off in ADC""s between speed and dynamic range. That is, dynamic range is typically limited by noise and offset errors, both of which tend to increase with speed.
Accordingly, there is a need for improved inspection mechanisms that are capable of quickly detecting light having a relatively high dynamic range.
Accordingly, mechanisms are provided for detecting a relatively wide dynamic range of intensity values from a beam (e.g., scattered light, reflected light, or secondary electrons) originating from a sample, such as a semiconductor wafer. In other words, the inspection system provides detected output signals having wide dynamic ranges. The detected output signals may then be analyzed to determine whether defects are present on the sample. For example, the intensity values from a target die are compared to the intensity values from a corresponding portion of a reference die, where a significant intensity difference may be defined as a defect.
In a specific embodiment, an inspection system for detecting defects on a sample is disclosed. The system includes a beam generator for directing an incident beam towards a sample surface and a detector positioned to detect a detected beam originating from the sample surface in response to the incident beam. The detector has a sensor for detecting the detected beam and generating a detected signal based on the detected beam and a non-linear component coupled to the sensor. The non-linear component is arranged to generate a non-linear detected signal based on the detected signal. The detector further includes a first analog-to-digital converter (ADC) coupled to the non-linear component, and the first ADC is arranged to digitize the non-linear detected signal into a first digitized detected signal. The system further includes a data processor for determining whether there is a defect present on the sample surface based on the first digitized detected signal.
In a further aspect, the system also includes a transformation mechanism for transforming the first digitized detected signal into a second digitized detected signal that compensates for noise variation associated with different intensity levels of the first detected output signal. The data processor is further arranged to receive the second digitized signal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the second digitized detected signal. In a further implementation, the transformation mechanism operates to cause a derivative of the second digitized detected signal to be equal to a normalization function, which is an estimate of the inverse of the noise level or uncertainty in the measurement. One such normalization function may be computed by dividing an average of an envelope function by the envelope function itself, the envelope function being calculated based on an observed repeatability of measurements of the first digitized detected signal.
In one aspect, the sensor is a photomultiplier tube (PMT). In other implementations, the sensor is an electron multiplier tube, a micro-channel plate PMT, an avalanche photodiode, a metal channel dynode PMT, a wire mesh dynode PMT, a PMT with explicit gate or grid electrodes, or an imaging array with programmable integration time.
In one aspect, the non-linear component is a logarithmic amplifier. In a further aspect, the detector further includes a first feed back circuit for automatically adjusting a sensor gain of the sensor based on the non-linear detected signal or the detected signal. In one embodiment, the first feed back circuit has a variable voltage supply component coupled to the non-linear component and arranged to adjust a voltage level of the sensor gain based on non-linear detected signal or the detected signal, a voltage reference signal, and one or more control signal(s). The first feed back circuit further includes an amplifier coupled to the variable voltage supply and arranged to amplify the sensor gain signal prior to it being input to the sensor.
In a further embodiment, the system further includes a second ADC for receiving the sensor gain, digitizing the sensor gain and outputting it as a digitized sensor gain signal. The system also has a first transformation mechanism for calibrating the digitized detected signal into a calibrated detected signal and a second transformation mechanisms for calibrating the digitized sensor gain signal into a calibrated gain signal. The system further includes an arithmetic logic unit (ALU) arranged to subtract the calibrated gain signal from the calibrated detected signal to form a first detected output signal. Preferably, the first and second transformation mechanisms take the form of a look-up table embodied within a memory device, but they may also be implemented as a mathematical equation which is evaluated by a digital computer, digital signal processor, or programmable logic device. The data processor is further arranged to receive the first detected output signal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the first detected output signal.
In yet a further aspect, the system includes an offset mechanism arranged to receive a user-selected sensor gain and offset the first detected output signal by a log of the user-selected gain to thereby emulate a programmable sensor gain, where the sensor gain is not altered. In a further aspect, the first and second transformation mechanisms and the ALU have a higher resolution than the first and second ADCs, in order to avoid rounding errors in the transformations.
In another aspect, the system includes a second linear or non-linear amplifier (e.g., a logarithmic amplifier) arranged to receive an illumination level of the incident beam, followed by a third ADC and third transformation mechanism which determines the logarithmic value of the illumination level to produce a log illumination level. The ALU is further arranged to subtract the log illumination level from the first detected signal. In a further embodiment, the system includes a second feed back circuit for automatically adjusting the illumination level based on the sensor gain, the non-linear detected signal or the detected signal.
In yet another aspect, the system includes a third transformation mechanism for transforming the first detected output signal into a second detected output signal. The second detected output signal is a relinearized first detected output signal when a mode signal input to the third transformation mechanism indicates a first mode and the second detected output signal equaling the first detected output signal when the mode signal indicates a second mode. In a further embodiment, the second detected output signal equals a noise compensating transformation of the first detected output signal when the mode signal indicates a third mode. The data processor is further arranged to receive the second detected output signal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the second detected output signal.
In an alternative embodiment, an inspection system for detecting defects on a sample is disclosed. The system has a beam generator for directing an incident beam towards a sample surface and a detector positioned to detect a detected beam originating from the sample surface in response to the incident beam. The detector includes a sensor for detecting the detected beam and generating a detected signal based on the detected beam, a logarithmic amplifier coupled to the sensor and arranged to generate a logarithmic detected signal based on the detected signal, and a first analog-to-digital converter (ADC) coupled to the logarithmic amplifier, the first ADC being arranged to digitize the logarithmic detected signal into a digitized detected signal. The detector also includes a first look-up table embodied in a first memory device and arranged to calibrate the digitized detected signal into a calibrated detected signal and a feed back circuit for automatically adjusting a sensor gain of the sensor based on the logarithmic detected signal or the detected signal, the sensor gain being input to the sensor. The detector also includes an amplifier arranged to amplify the sensor gain to an amplified sensor gain and a second ADC coupled to the amplifier and arranged to digitize the amplified sensor gain into a digitized sensor gain. The detector also has a second look-up table embodied in a second memory device and arranged to calibrate the digitized sensor gain signal into a calibrated sensor gain signal and an arithmetic logic unit (ALU) arranged to subtract the calibrated gain signal from the calibrated detected signal to form a first detected output signal.
In a further implementation, the system includes a third look-up table embodied in a third memory device and arranged to transform the first detected output signal into a second detected output signal to facilitate data processing and a data processor arranged to analyze the second detected output signal to determine whether there is a defect on the sample surface. In a further implementation, the second detected output signal is a relinearized first detected output signal when a mode signal input to the third look-up table indicates a first mode and the second detected output signal equaling the first detected output signal when the mode signal indicates a second mode. In a further embodiment, the second detected output signal equals a noise compensating transformation of the first detected output signal when the mode signal indicates a third mode. In a final aspect, the inspection system includes an offset mechanism arranged to receive a user-selected sensor gain and offset the first detected output signal by a log of the user-selected gain to thereby emulate a programmable sensor gain, where the sensor gain is not altered. The ALU is further arranged to add the log user-selected sensor gain to the first detected output signal.
In another embodiment, the invention pertains to a method for detecting defects on a sample. An incident beam is directed towards a first sample surface. A first detected beam is detected and a first detected signal is generated based on the first detected beam. The first detected beam originates from the first sample surface in response to the incident beam. A first non-linear detected signal is generated based on the first detected signal. The first non-linear detected signal is digitized into a first digitized detected signal, and the first digitized detected signal is analyzed to determine whether it corresponds to a defect on the first sample surface. A first sensor gain of the sensor is automatically adjusted based on the first non-linear detected signal or the first detected signal.
In a further aspect, an incident beam is directed towards a second sample surface. A second detected beam is detected and a second detected signal is generated based on the second detected beam. The second detected beam originates from the second sample surface in response to the incident beam. A second non-linear detected signal is generated based on the second detected signal, and the second non-linear detected signal is digitized into a second digitized detected signal. A second sensor gain of the sensor is automatically adjusted based on the second non-linear detected signal or the second detected signal.
The first and second digitized detected signals are logarithmic values. The first digitized detected signal is analyzed by subtracting the second digitized detected signal, the subtraction resulting in a difference of a log of intensity values from the first and second sample surfaces, which corresponds to the log of the ratio of the intensities. It is determined that the first digitized detected signal corresponds to a defect on the first sample surface when the difference is above a predetermined threshold.
In an alternative embodiment, the inspection system includes a beam generator for directing an incident beam towards a sample surface and a beam splitter for receiving a detected beam from the sample surface which is responsive to the incident beam and splitting the detected beam into a first fraction and a second fraction, wherein the first fraction is significantly larger than the second fraction. The system further includes a high gain sensor for receiving the first fraction of the detected beam and generating a first detected signal based on the first fraction and a low gain sensor for receiving the second fraction of the detected beam and generating a second detected signal based on the second fraction. The system further includes a control block coupled with the low gain sensor and operable to regulate a gain of the high gain sensor based on the second detected signal and to output an invalid signal indicative of a reliability factor of the first detected signal, a first ADC for receiving the first detected signal, digitizing it, and outputting a first digital S detected signal, a second ADC for receiving the second detected signal, digitizing it, and outputting a second digital detected signal, and a data processor for receiving the first and second digital detected signals and the invalid signal and determining whether there is a defect present on the sample surface. The determination is based on the first digital detected signal when the Invalid signal indicates that the first digital detected signal is reliable and based on the second digital detected signal when the Invalid signal indicates that the first digital detected signal is unreliable.
In one aspect, the step of determining whether there is a defect present on the sample surface is accomplished by analyzing the first and second digital detected signals separately to determine whether there is a defect on the sample surface and reporting any defect found during the analysis of the first digital detected signal only when the Invalid signal indicates that the first digital detected signal is reliable. The determination is further accomplished by reporting any defect found during the analysis of the second digital detected signal only when the Invalid signal indicates that the first digital detected signal is unreliable.
In another aspect the step of determining whether there is a defect present on the sample surface is accomplished by selecting a first operating voltage for the high gain sensor and a second operating voltage for the low gain sensor so that a ratio of the effective gains of the high gain sensor and the low gain sensor is equal to the Mth power of two, where M is an integer. The determination is further accomplished by forming an output data word from the first digital detected signal by padding the first digital detected signal with M zeros on the most significant bit side when the Invalid signal indicates that the first digital detected signal is reliable. Otherwise, the output data word is formed from the second digital detected signal by shifting the second digital detected signal M bits towards the most significant bit side and padding the shifted signal with M zeros on the least significant bit side, when the Invalid signal indicates that the first digital detected signal is unreliable. In a specific implementation, the control block is operable to regulate the gain of the high gain sensor by automatically adjusting the gain of the sensor based on the second detected signal or the second fraction. In a further implementation the control block is operable to regulate the gain of the high gain sensor by turning off the high gain sensor or indicating to the high gain sensor that it should turn off when the second detected signal or the second fraction rises above a predetermined threshold and turning the high gain sensor back on or indicating to the high gain sensor that is should turn back on when the second detected signal or the second fraction falls back below the predetermined threshold.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.